The Automated Weather/Surface Observing System (AWOS/ASOS) is a suite of sensors which measure, collect and disseminate weather data to help meteorologists, pilots and flight dispatchers prepare and monitor weather forecasts, plan flight routes, and provide necessary information for correct takeoffs and landings. AWOS/ASOS installations provide a minute-to-minute update that is usually furnished to pilots by a VHF radio on a frequency between 118 and 136 MHz, by a telephone link, or through regular weather dissemination channels. AWOS/ASOS's are categorized as either Federal or Non-Federal. Federal AWOS/ASOS's were purchased and are currently maintained by the FAA. Non-Federal AWOS/ASOS's are purchased and maintained by state, local, and private organizations. The sensors measure weather parameters such as wind speed and direction, temperature and dew point, visibility, cloud heights and types, precipitation, and barometric pressure. AWOS/ASOS does not predict weather, but many send current information to weather offices where forecasts are produced using this information along with computer model outputs, satellite photos and radar images, to name a few.

Every hour on the hour, the AWOS data is made available to off-site users by those AWOS/ASOS's on Service A (long line telephone communication) or satellite uplink. Should conditions change rapidly, SPECIAL reports may also be made available. The aviation community, which is one of the largest users of environmental data, is the major user of the AWOS information.

AWOS/ASOS makes weather information available to pilots which was previously unavailable, except at airports with trained human weather observers. While the implementation of AWOS/ASOS has been helpful to pilots operating under Part 91 (most of us), it has been most important to those operators under Parts 135 and 121. Under those parts of the Federal Aviation Regulations, operators may not conduct instrument operations to airports without weather reporting, and may not even begin an instrument approach unless the reported weather is at or above minimums prior to reaching the final approach fix.

As a result, many airports which could not be reached by commercial operators are now accessible. More important, the availability of up-to-the minute weather information results in increased operational safety for all.

Typical AWOS/ASOS Report

METAR KCDW 270953Z AUTO 18019G25KT 3SM BR BKN007 BKN011 19/18 A2999 RMK AO2 CIG 005V009 SLP159 T01890183 TS DSNT SE MOV E TSB00E37

Wind direction and velocity is determined by a standard wind vane and anemometer installation on a pole near the AWOS/ASOS installation. This is the setup at Sussex, New Jersey.

To the left is the wind vane, and to the right is the anemometer. The rod sticking up in the middle is a lightning rod. The data developed is transmitted by wire to a data processing unit at or near the AWOS/ASOS installation.

AWOS/ASOS winds are treated as TOWER winds and are thus reported in degrees referenced to MAGNETIC North, and in the stated speed format.

The time averaged prevailing visibility is determined through the use of a Xenon flash forward scatter system consisting of a transmitter and a receiver, Belfort Model 6220. Each unit is pointed slightly downward and offset so that the flash does not directly go from the transmitter to the receiver, but rather to a 9 inch spot in the atmosphere somewhere in between. This sensor unit is elevated above the surface and is located at Sussex Airport, New Jersey. The shot was taken facing Northwest; the transmitter is at the right and the receiver is at the left. The flash is visible to the observer. The wind instruments are visible in the background.

In practice, the transmitter unit sends a flash of light. The light reflects off particles in the air, and is sensed by the receiver unit. The amount of reflected light that is picked up by the receiver unit is sent to the computer and converted to Sensor Equivalent Visibility. The greater the number of reflective particles in the air, the greater the amount of light reflected, and the lower the Sensor Equivalent Visibility.

The type of precipitation, if any, falling at an AWOS/ASOS installation is determined by a device known as a Precipitation Discriminator. It is actually called a Light Emitting Diode Weather Indicator (LEDWI).

The unit operates on the principle that different types of precipitation have differing values of reflectivity to light. The discriminator operates similarly to the unit which generates visibility values. In this case, however, the transmitter and receiver are pointed at each other. The data generated is sent to the computer which determines the type of precipitation which is falling, if any.

The amount of liquid precipitation is determined through the use of a tipping bucket made by Frise Engineering of Baltimore.

The blue skirt around the unit is a wind skirt, while the white cylinder in the center contains the measuring apparatus.

In practice, the precipitation enters the top of the cylinder through a funnel which directs it to a one of a pair of buckets which can hold .01 inch of liquid. As a bucket fills to capacity, it tips, empties, and places the other bucket in a position to fill. As the tipping occurs, a data pulse is sent to the computer to register .01 inch of precipitation.

This picture is of the ASOS CEILOMETER Located at Sussex Airport, New Jersey. Behind it you can see the freezing precipitation detector.

The ceilometer shoots a series of light pulses vertically and sends the received data to the computer. The ceiling algorithm converts the reading to a time averaged report of cloud heights and coverage.

This device is used to determine temperature and dewpoint, the values of which are very important in day-to-day aviation operations. Temperature is determined with an electronic thermometer. Unlike in the past, when a human observer determined the dewpoint through the use of a sling psychrometer (or still does), the AWOS/ASOS uses an electronic means to determine this value.

Very simply, a fan draws ambient air into the assembly where temperature and dewpoint are measured. Dewpoint is determined by drawing the air over a mirror which reflects light form a source to a sensor. The temperature of the mirror is cooled through a refrigeration process until moisture condenses on it, reducing the light reflected to the sensor. The temperature at which this occurs is measured and is the dewpoint.

As reflected in an automated METAR, the temperature and dewpoint are presented to the nearest degree, with the exact readings to a tenth of a degree presented near the end of the METAR format.

These pieces of information are developed by standard electronic means, and are adjusted to sea level. In most places in the US, the altimeter setting is presented in inches of mercury, and the Sea Level Pressure is presented in millibars. Rapid changes in pressure are reported.

Thunderstorm detection is through standard sferics devices, similar to Stormscopes or Strikefinders. Based upon lightning strike data received, a report is made of the estimated location and movements of thunderstorms.

The use of AWOS/ASOS provides weather information where it might not be available, with some limitations. Most of the limitations of automated reporting revolve around the fact that the unit can only sample atmospheric conditions in a small area. While the sensors and computer algorithms used have been configured to provide maximum accuracy, the fact is that automated reporting can never be quite as good as the data from a human observer, particularly as regards ceiling and prevailing visibility.

An automated unit is only able to sample conditions in a small area, and it is unable to "see" beyond this area. For example, an automated unit may be enveloped in fog, but the runway environs may be clear. The unit cannot develop a figure for prevailing visibility as can a human observer, so you sill not see visibility data reflecting different visibilities in different directions.

Used with its limitations in mind, AWOS/ASOS provides valuable information and allows us to achieve a higher level of safety as compared to the days when we operated without weather reporting at the majority of airports.